diff --git a/docs/conf.py b/docs/conf.py
index 16570c35c3..1ce653e4ce 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -169,9 +169,12 @@ def _warn_node(self, msg, node, **kwargs):
 
 # Add custom .css files
 # https://github.com/snide/sphinx_rtd_theme/issues/117#issuecomment-41571653
-def setup(app):
-   app.add_stylesheet("custom.css")
-   app.add_stylesheet("rtfd-css.css")
+# 2022-06-22: move to `html_css_files` as `add_stylesheet` is deprecated/removed
+#   https://github.com/sphinx-doc/sphinx/issues/9683#issuecomment-932878909
+html_css_files = [
+   "custom.css",
+   "rtfd-css.css",
+]
 
 # Add any extra paths that contain custom files (such as robots.txt or
 # .htaccess) here, relative to this directory. These files are copied